This invention relates to variable speed ac motor drives implemented with controlled current or voltage-fed inverters, and more particularly to an inverter frequency control having a reversible variable frequency oscillator capable of inherent multipulse operation for smooth reversing of the motor drive.
Current inverter drives utilizing load angle control are one of the several different types of static adjustable frequency ac motor drives. In these systems at low speed operation the desired frequency is the small dc component of the inverter frequency command signal which also has a large amplitude sixth harmonic ripple. It is not feasible to filter this signal and maintain required system dynamic performance. Both the desired frequency and sense of motor rotation, forward or reverse, must be determined from the signal, and operation and reversing must be smooth over a wide torque range. Without special effort when using angle control, low speed operation and reversing can be very unstable in that the drive can fall into a mode of repeated reversals which yield totally unsatisfactory system performance and which can damage the mechanical load. Load angle control of a controlled current inverter drive and pulse width modulated voltage inverter drive is disclosed and claimed in allowed application Ser. No. 729,042, (now U.S. Pat. No. 4,088,934 issued May 9, 1978) filed on Oct. 4, 1976 by T. A. Lipo, A. B. Plunkett, and J. D. D'Atre and assigned to the assignee of this invention. For the transportation application being developed, the problem was solved by means of a diode clipper which limited the dynamic range of the angle error signal prior to inputting to a proportional plus integral controller. The present drive system developed for another application does not contain an integration of the angle error signal and it was determined that the simple diode clamp was not a solution.
The operation of controlled current and square wave voltage inverters in a pulse width modulated mode, with the timing of the pulses optimized mathematically to eliminate various harmonics, has been described in the prior art. The usual means of timing these pulses, however, is to generate a proper pattern at signal level using digital logic in a counting mode. Multipulse operation can reduce or eliminate detrimental cogging torque at low speeds, but previous attempts to combine multipulse operation with load angle regulation have not been successful.